Posttreatment Response Assessment Using 18 F-FDG PET in Salivary Gland Carcinoma Patients Treated With Definitive Radiation Therapy—A Multicentric Retrospective Study

Abstract

Presence of residual soft tissue lesions in the post-treatment anatomical imaging studies is frequently observed in salivary gland carcinoma (SGC) patients treated with definitive radiotherapy. This report aims to investigate the clinical utility of 18F-FDG PET in the post-radiotherapy response evaluation in SGC patients. We retrospectively reviewed the medical records of 35 SGC patients who were treated with definitive radiotherapy and received post-treatment 18F-FDG PET scan between 2004 and 2014. Adenoid cystic (N = 13) and mucoepidermoid (N = 9) carcinomas were the most common histology subtypes. Most patients (N = 24, 68%) had minor salivary gland malignancies, and parotid, submandibular, and sublingual gland tumors were documented in 8 (23%), 2 (6%), and 1 (3%) patients, respectively. Thirty-four (97%) patients also received pretreatment 18F-FDG PET. Pre- and post-treatment maximum standardized uptake values (SUVmax) were measured. The median radiation dose was 72 Gy (range, 66-80). Concurrent chemotherapy was performed in 33 (94%) patients. The median follow-up time for the survivors was 46 (range, 26-148) months. In the post-radiotherapy anatomical images (CT or MRI), 22 (63%) patient presented residual soft tissue [partial response (PR), N = 16; stable disease (SD), N = 3; progressive disease (PD), N = 3], and complete response (CR) was recorded in 13 (37%) patients. The post-radiotherapy 18F-FDG PET images documented metabolic CR, PR, SD, and PD in 20 (60%), 8 (22%), 3 (9%), and 3 (9%) patients, respectively. In addition, the metabolic CR was more likely to be observed in patients with a pretreatment low-SUVmax (≤7.4) tumor than in those with a high-SUVmax tumor (>7.4) (78% vs 50%, P = 0.091). The metabolic complete responders had significantly higher 5 year locoregional control (86% vs 42%, P = 0.007), distant metastasis-free survival (87% vs 46%, P = 0.003), disease-free survival (83% vs 27%, P = 0.002), and overall survival rates (96% vs 40%, P = 0.003) rates, compared with the metabolic non-complete responders (PR+SD+PD). Nearly two third of SGC patients presented residual soft tissue lesions in the anatomical images after definitive radiotherapy, and post-treatment 18F-FDG PET is effective to differentiate responders from patients resistant to the radiation treatment.